D is the best answer.

In lines 17-24, the author describes the reasoning behind the multioccupational unions’ attempt to achieve a fully unionized workplace. This reasoning is provided to explain why “the multioccupational unions would often try to organize them (clerical workers) regardless of the workers’ initial receptivity”(lines 15-17). A helps to explain, but is not explained by, the attempt to achieve a fully unionized work force. An explanation for C is given in lines 4-7. B is explained in the second paragraph of the passage.